Entry into Canada: Canadian law requires that all persons entering Canada carry both proof of citizenship and proof of identity. A valid U.S. passport or passport card satisfies these requirements for U.S. citizens.
Children under 16 need only present proof of U.S. citizenship.
Entry into the United States: When traveling by air from Canada, U.S. citizens are required by U.S. law to present a U.S. passport book, except as noted in the few exceptions provided on the U.S. Customs and Border Protection website.

For entry into the United States via land and sea borders, U.S. citizens must present either a U.S. passport or a passport card. The only exception to this requirement is for U.S. citizens under the age of 16 (or under 19, if traveling with a school, religious, or other youth group) who need only present a birth certificate, Consular Report of Birth Abroad, or naturalization certificate.

Travel with Minors: If you plan to travel to Canada with a minor who is not your own child or for whom you do not have sole, full legal custody, you may be required to present a notarized affidavit of consent from the minor’s parent(s) stating they are aware their child is traveling with you. There is no specific form for this document, but it should include beginning and ending dates of travel, Child/children’s full legal name and date of birth, relationship of child/children to you, full names and relationships of adult(s) traveling with the child/children. Your full legal name(s)emergency contact number and photocopies of your state-issued IDs.